It's going to be costly. A lot depends on your lifestyle and how much you want to spend/save between trips. Kangsan are located in Busan and they always seem to have competetive prices.Their website is : http://kangsantravel.com As always though, it's best to make a call to travel agents in Korea, rather than e mail if possible.They are more responsive that way. I'd also mention that I plan on taking the trip regularly, it may help.
For more info on Busan : http://www.pusanweb.com/